1. Field of the Invention
The invention is related to a transmission system comprising a transmitter for transmitting simultaneously a plurality of tone signals via a transmission medium to a terminal, the terminal comprising strength measuring means for deriving tone strength measures for said plurality of tone signals.
The invention is also related to a terminal, a tone detector and a method for tone detection.
2. Description of the Related Art
A transmission system according to the preamble is known from U.S. Pat. No. 3,882,283.
The transmission system according to the present invention can be applied in systems using multi tone transmission. This means that information is transmitted by presenting simultaneously a plurality of tone signals to a channel. The receiver can recover the information by detecting the presence of a combination of tone signals.
A first application of such a transmission system is the so-called DTMF signalling system, which is in widespread use for transmitting dialling information from a telephone terminal to a telephone exchange. In this system a digit is transmitted by presenting two tone signals to the transmission medium. One of said tone signals is chosen from a first group of four tone signals and the second tone is chosen from a second group of four tone signals. The frequencies of the tone signals in the first group (low frequency group) are lower than the frequencies of the tone signals in the second group (high frequency group). The receiver has to determine which pair of tone signals is transmitted by the receiver in order to recover the transmitted digit.
A further application of the present invention is the detection of the so-called CAS tone in screen phones. Recently, telephone terminals provided with a display have become available. These telephone terminals are arranged to operate according to the Analog Display Service Interface standard (ADSI). Said standard enables the transmission of data in addition to the transmission of speech signals to and from the telephone terminal.
In order to distinguish between speech and data, a so called CAS-signal (CPE Alerting Signal) is transmitted by the transmitter to indicate that a data signal will be transmitted. The CAS signal is constituted by two tone signals having a frequency of 2130 Hz and 2750 Hz, which are transmitted simultaneously during 80 msec.
The terminal of the transmission system according to the above mentioned U.S. patent, comprises for each of the tone signals to be detected a strength measuring device which generates a tone strength measure for the corresponding tone. This tone strength measure can e.g. be the amplitude or the power of the tone. If the strength measure of a tone exceeds a threshold, said tone is regarded to be present. A predetermined tone combination is regarded as present if all individual tone signals of this combination are regarded to be present.
Due to imperfections of the transmission medium, it can occur that the individual tone signals at the receiver differ in strength. The ratio between the strength of the individual tone signals is often referred to as twist. In order to be able to detect a combination of tone signals having some twist, the threshold for detecting the presence of a tone signal has to be lowered. This lowering of the threshold results into an increased probability of regarding noise signals as the tone to be detected, making the detection of the tone signal combination less reliable.